This invention relates to a process for preparing a microporous, low density foam and to foams made by the process.
Processes for the production of open-celled polymeric foam from a solution of polymer in a suitable solvent have been known for many years. Many of these processes involve phase inversion from a solution. In such a process, a homogenous polymer solution is cooled in order to bring about separation of the solvent from the polymer to form a continuous polymer matrix in which is dispersed discrete cells of solvent. Typically, the solvent is selected from a class of solvents which are good solvents at one temperature and poor solvents at a lower temperature. Controlled cooling of a solution comprising a polymer and such a solvent yields a polymer foam which, once the solvent has been removed by, e.g., freeze-drying, comprises essentially only polymer.
In general, such processes heretofore known in the art are not capable of producing polymeric foams of relatively high volume having both a small cell size and low density.
U.S. Pat. No. 4,519,909 discloses a process for the manufacture of microporous polymers. The process includes forming a solution of a synthetic thermoplastic polymer and a compatible liquid, allowing the solution to form a desired shape and subsequently cooling the solution in that shape at a rate and to a temperature such that thermodynamic non-equilibrium liquid-liquid phase separation is initiated. The cooling is continued until a solid results and, finally, at least a substantial portion of the liquid is removed from the resulting solid to form a microporous polymer. Although the process results in foams having relatively small cell size, the patent does not disclose foams having both a small cell size and low density. The patent discloses that the rate of cooling affects the absolute cell size of the resultant foam and that high cooling rates can be employed to produce a foam having a small cell size. However, in each instance where high cooling rates are achieved, only very small samples of solution can be cooled. Additionally, since the samples are cooled in "pans", the solution is not cooled in one-dimensional fashion. The pans do not have side and top walls of lower thermal conductivity than the bottom walls. Accordingly, substantial cooling through the side and top walls occurs. U.S. Pat. No. 4,247,498 presents a similar disclosure.
U.S. Pat. No. 3,308,073 discloses a process for the manufacture of porous polyolefin foams. This process does not result in a foam having both low density and small cell size. Additionally, the process cannot be used to produce foams from atactic polymers. Thus, e.g., polystyrene foams cannot be produced by this process if sublimable solvents are used.
U.S. Pat. No. 3,812,224 discloses another process for the production of porous polymeric foams wherein a solution comprising a polymer and a compatible liquid solvent is frozen to a crystalline state and the solvent is removed by freeze-drying. The resultant foam contains interconnecting cells which are not greater than 1,000 microns in width. Low density foams cannot be made by this process.
Foams having both a small cell size and relatively low density have been produced from specific polymers. For example, U.S. Pat. No. 4,430,451 discloses a microcellular, low density foam comprising poly(4-methyl-1-pentene) having a density of from about 10 to about 100 mg/cc and cell sizes of from about 10 to about 300 micrometers. The foam is produced by a phase inversion process simliar to those discussed above. However, foams having both low density and small cell size can be produced only if the polymer used is isotactic and only if a relatively viscous gel phase forms upon cooling. This gel phase is due to the isotactic nature of the polymer since isotactic polymers readily crystallize. Thus, the polymer itself forms an immobile phase which results from the characteristics of the polymer, not the process parameters employed. U.S. Pat. No. 4,118,449 discloses a similar process for the production of foams having a density of from 0.065 to 0.6.times.10.sup.3 kg/m.sup.3 and a cell size ranging from about 0.3 to about 2 microns. However, such processes cannot be used to produce foams having a small cell size and low density from polymer solutions which do not form a gel when cooled, e.g., polystyrene. Low density polystyrene foams made by conventional processes have cell sizes of 100-200 .mu.m or greater.
As disclosed in "Preparation of Multishell ICF Target Plastic Foam Cushion Materials by Thermally Induced Phase Inversion Processes", J. Vac. Sci. Tech. 20 (4), April 1982, 1094-1097, Young, et al, water soluble, hydrogen bonding polymers (carboxymethylcellulose and dextran) can be used to prepare foams having a density of 50 mg/cc and an average cell size of 2 micrometers can also be produced by such processes.
All of the above references are hereby specifically and entirely incorporated by reference herein.
In general, low density foams have been difficult to produce because such foams are very fragile and large bodies of such foams have low structural integrity.